1. Field of the Invention
The present invention relates to a method for manufacturing oxidic nuclear fuel bodies with an oxygen-to-metal ratio of 2.0.+-.0.02 at sintering temperatures between 1000.degree. and 1400.degree. C.
2. Description of the Prior Art
In the manufacture of nuclear fuel bodies, it is the general practice today to sinter them in a reducing atmosphere at 1700.degree. C. Hydrogen or cracked ammonia is used as the reducing atmosphere. Sintered bodies of nuclear fuel materials must attain high operating reliability during the irradiation in the nuclear reactor; they must therefore meet special requirements. These requirements are in substance: The sintered density of the molded bodies is to be higher than 93% of theoretical density; the microstructures should be stable against densification; and the fission gas liberation should be limited; for reasons of corrosion, the fluorine contents should be less than 10 ppm; and an oxygen-to-metal ratio of 2.0.+-.0.02 should be maintained.
Since the present furnace temperatures of 1700.degree. C. represent an extraordinarily high stress for the furnace insulation and the furnace lining as well as for the heating elements, which limits the service life seriously, it would be highly desirable if sintering temperatures were sufficient which are substantially lower. Therefore, various proposals in this respect have been made. According to a proposal of U.S. Pat. No. 3,375,306, the pressed nuclear fuel powder is sintered in gas mixtures of CO.sub.2 and CO up to 95% of its theoretical density at a temperature of 1300.degree. to 1600.degree. C. The reduction of the overstoichiometrically sintered molding is accomplished during the cooling in hydrogen or in mixtures of CO.sub.2 and CO. A method described in U.S. Pat. No. 3,927,154 works likewise with CO.sub.2 /CO mixtures and sintering temperatures in the range of 1000.degree. to 1400.degree. C. This method represents in substance sintering operation with continuously changing oxygen-to-metal ratio in the sintered body. The oxygen potential present in the sintering atmosphere is low, so that from the moment of reaching the sintering temperature on, the oxygen-to-metal ratio of the sintered body decreases slowly and finally should reach the specified value .ltoreq.2.02. Thus, this method depends heavily on the oxygen-to-metal ratio of the starting powder. Unfortunately, these sintering methods have not found acceptance in practice.